Transmitters form a significant portion of most communication circuits. As such, they assume a position of prominence in design concerns. With the proliferation of mobile terminals, transmitter design has progressed in leaps and bounds as designers try to minimize components and reduce size, power consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers within limited bandwidth constraints. Changes in standards or standards based on newly available spectrum may also cause designers to approach the design of transmission systems with different modulation techniques.
Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). GSM comes in many flavors, not the least of which is General Packet Radio Service (GPRS). GPRS is a new non-voice value-added service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data and Short Message Service. GSM allows many different types of mobile terminals, such as cellular phones, pagers, wireless modem adapted laptops, and the like, to communicate wirelessly through the Public Land Mobile Network (PLMN) to the Public Switched Telephone Network (PSTN).
One relatively recent change has been the advent of the Enhanced Data for GSM Evolution (EDGE) scheme in GSM systems. This system contains amplitude modulation components, and, as a result, the power amplifier must be linear and should not operate in saturation when classical modulation techniques are employed. Such a linear system lacks the efficiency of one that operates the power amplifier in saturation.
If a polar modulation system is used instead of a classical modulation system, then the power amplifier may operate in saturation and efficiency would be greatly improved. In addition, if the polar signals are generated by a digital method, such a system does not require the use of a high current drain quadrature modulator. Quadrature modulators are undesirable from a design standpoint in that they draw large amounts of current, and hence, drain batteries comparatively fast.
Unfortunately, further complicating matters, the amplitude signal that controls the power amplifier will cause unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called amplitude to phase (AM to PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude. Thus, a need also exists to be able to counteract or eliminate the unwanted AM to PM distortion of the transmitted phase signal.
An additional concern is that the power amplifier may have a non-linear gain with varying output power. This may create what is called amplitude to amplitude (AM to AM) distortion. The AM to AM distortion may have both phase and amplitude distortion components, and to create a better control system, these should be reduced or eliminated as well.
One solution used to counteract AM to AM and AM to PM distortion is to measure the AM to AM and AM to PM distortion of the power amplifier, and then create a polynomial for amplitude pre-distortion and a polynomial for phase pre-distortion that are used to distort a signal prior to amplification by the power amplifier. This pre-distortion offsets the AM to AM and AM to PM distortion of the power amplifier. However, the polynomials are often at least third order polynomials and are costly to implement. Further, the polynomials may not effectively offset the AM to AM and AM to PM distortion of the power amplifier at every power level of the power amplifier. Thus, there remains a need for a more cost effective system for correcting AM to AM and AM to PM distortion of a power amplifier at every power level of the power amplifier.